Gas insulated electrical equipment has a cylindrical metal enclosure with a cylindrical high voltage conductor disposed therein such that it is coaxial with the metal enclosure. The metal enclosure has an insulating gas introduced therein. The insulating gas is mainly SF6 gas, dry air, nitrogen gas, carbonic acid gas, CF4 gas, CHI5 gas, C2F6 gas, C3F8 gas and/or the like singly or mixed together.
In particular, SF6 gas has a dielectric strength about 3 times that of air, and accordingly, it is used as an insulating gas allowing the equipment to have a high voltage unit and a ground electrode with a reduced distance therebetween to miniaturize the equipment.
The gas insulated electrical equipment typically has the insulating gas compressed and thus used for enhanced insulation. In order to ensure a fixed distance to the high voltage conductor for insulation while sealing the insulating gas, the gas insulated electrical equipment is structured to have the cylindrical high voltage conductor in the cylindrical metal enclosure coaxially, as described above.
When the insulating gas is SF6 gas, it is necessary to consider that its insulation property may be impaired in an unbalanced electric field. For example, if the gas insulated electrical equipment is a switch, a metallic particle of a size of the millimeter level may be generated from those portions of metals which slide on each other, and those portions of conductors of a breaker, a disconnecting switch or the like.
When the metallic particle is generated, then, in an initial stage, the metallic particle deposits on the bottom of the metal enclosure. The metallic particle is acted on by static induction or the like and thus gradually, electrically charged, and the metallic particle starts to reciprocate such that it repeatedly ascends from and descends to the bottom of the metal enclosure in accordance with an electric potential gradient provided between the metal enclosure and the high voltage conductor while the gas insulated electrical equipment is in operation.
While the metallic particle is electrically less charged, the metallic particle reciprocates in a vicinity of the bottom of the metal enclosure, however, as the metallic particle is increasingly, electrically charged, the metallic particle ascends to higher levels and thus comes close to or into contact with the high voltage conductor.
In a vicinity of the high voltage conductor, there is a highest electric field. Accordingly, when the metallic particle approaches the high voltage conductor, an electric field concentration is caused in a vicinity of the metallic particle, which results in an unbalanced electric field distribution and causes an electric discharge. When the electric discharge is caused, a flashover may be caused via the metallic particle and result in destroying the entire circuitry.
Flashover via a metallic particle can be minimized by configurations disclosed in prior art documents including Japanese Patent Laying-Open No. 2009-284651 (PTD 1) and Japanese Patent Laying-Open No. 5-30626 (PTD 2).
PTD 1 describes a sealed insulation device including a metal enclosure having an internal surface provided with a nonlinear resistance film that is provided to be high in electric resistance when the metal enclosure's internal surface is acted on by an electric field having a critical value or lower and to be low in electric resistance when the metal enclosure's internal surface is acted on by an electric field higher than the critical value.
PTD 2 describes a composite insulated bus including a high voltage conductor and a metal enclosure having an external surface and an internal surface, respectively, coated with a fluororesin coating of 10 mm or larger in thickness.